CMOS active pixel image sensors may be operated using a “rolling” shutter. Such a shutter operates by reading out each row of pixels, and then resetting that individual row, and then rolling to read and then reset the next row of pixels. Each pixel hence gets read and then reset at slightly different times. Hence, each pixel has a slightly different time of integration. Some applications, such as high-speed photography, may require more time consistency than is possible using this approach. Therefore, in these other applications, a frame shutter may be used. In the frame shutter mode, all pixels in the array have substantially identical integration start times and integration stop times.
A frame shutter technique, using a photogate pixel with two transfer gates, is shown in FIG. 1. A bridge diffusion isolates the photogate PG from the floating diffusion sense node (FD). Hence the charge in the photogate may be integrated and reset independently. The charge is stored in a storage node 110, which may also be independently reset. Bridge diffusion BD2 isolates the photogate from the storage node. A second bridge diffusion BD1 isolates the photogate collection area from a reset level Vaa. The charge level in the photogate collection area 100 may be transferred to a storage node 110 via transfer gate 105.
This isolation allows the integration regions of all the photogates in the entire array to be clocked globally. The same timing therefore exists for each of the plurality of pixels in the array. Moreover, the charge is preserved until the desired time of readout. This enables the active pixel sensor can be read out row by row, thereby allowing the same readout structure as is currently used.